Can you take a note of the time of day, then check the application and system event logs for any errors during that time? It may be hardware that is slow or failing to initialise.
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PaulMay 28 '12 at 1:07

Does it show up quicker going into Safe Mode? Are you on a network? Does unplugging peripheral devices help?
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Ƭᴇcʜιᴇ007May 28 '12 at 1:29